1. Field of the Invention
This invention pertains generally to the rapid transmission of fluid pressure signals through pressure control lines and more particularly to the rapid actuation of pilot valves through a long fluid control line to operate a plurality of subsea oil well valves in a predetermined sequential manner.
2. Description of the Prior Art
In a fluid pressure control system, various mechanical equipment is actuated by applying a fluid pressure signal to the equipment through a fluid-filled conduit. More specifically, hydraulic control systems have been used in the oil production industry to actuate valves and other devices connected to an oil well. Such systems have been applied to underwater well installations to open and close subsea valves on the well by introducing a hydraulic signal into the control system from a remote source on the ocean surface. The subsea valves may be Christmas tree valves located on the ocean floor, safety valves located downhole beneath the ocean floor or manifold valves in a subsea production system. The remote pressure source may be a pump and hydraulic accumulator unit fixed on a platform on the ocean surface at some distance from the well and connected to the underwater devices through a long control line.
In the operation of several subsea well valves it is often desired to open or close each valve in a particular order. A control system, including sequenced pilot valves, is commonly used to accomplish this purpose. Each pilot valve is connected to a hydraulic power line leading to one of the well valves and controls the flow of power fluid to the well valve. The pilot valves are hydraulically actuated by input pressure signals applied through a pressure control line. Each pilot is triggered by a different minimal output pressure in the control line at the location of the pilots. Thus, the well valves can be operated in the desired sequence by applying incremental pressure steps to the control line to yield progressively increasing pressure levels at the pilots. Typically, in the conventional method, each input pressure step in the sequence is about 400 psi (28.1 kg/cm.sup.2) and the increase in output pressure required to reach the next actuation pressure in the sequence is about 60% of the step or about 240 psi (16.9 kg/cm.sup.2). It is critical that the step size and the pilot actuation settings be chosen so that, during the application of a certain pressure step to actuate one pilot, the pilot associated with the next step in the sequence is not inadvertently actuated. The operation of a pilot valve is described in U.S. Pat. No. 4,119,146. The use of pilot valves to control devices in a predetermined sequence is discussed in U.S. Pat. Nos. 3,856,037 and 3,993,100.
One shortcoming of the conventional fluid pressure control method is the delay between the time at which the pressure signal is applied to the conduit at the pressure source and the time at which the signal reaches the device and causes actuation. This time delay becomes particularly significant when a long pressure line is used as in the case of subsea well valves controlled from a remote surface facility. In such installations, the control line may be more than twenty kilometers in length and the system response time may be several minutes.
In the past, the time delay problem has been recognized and various solutions have been proposed. By increasing the inside diameter of the pressure conduit in a given system, the response time for transmitting a signal will be reduced. A drawback of this proposal, however, is the additional expense associated with providing a larger pump and a larger accumulator to accommodate the increased fluid volume. Another proposed solution is the use of a water-based fluid rather than an oil as the hydraulic fluid. While the system response time will be reduced due to the lower viscosity of a water-based fluid, the corrosion problems associated with the fluid system may be increased, wear may be accelerated and fluid life will be reduced.